1. Field of the Invention
This invention relates to a magneto-optical recording apparatus for recording signals by applying a magnetic field modulated in conformity with a recording signal to a magneto-optical recording medium while applying a laser beam to the magneto-optical recording medium.
2. Related Background Art
In recent years, studies of an optical information recording apparatus as a memory of large capacity have been actively done. Particularly, magneto-optical recording has been drawing attention, because of its capability of rewriting information. As a method of magneto-optical recording, there is known a method of applying a magnetic field modulated in conformity with a recording signal to a magneto-optical recording medium while applying a light beam to the magneto-optical recording medium. This method is called a magnetic field modulation method, and includes a feature that signals to be renewed can be overwritten without erasing previously recorded signals in advance.
An example of a magneto-optical recording apparatus using the above-described method is schematically shown in FIG. 1 of the accompanying drawings. In FIG. 1, the reference numeral 1 designates a magnetooptical disk which is formed by attaching vertically magnetized film 3 to a glass substrate 2. This magnetooptical disk 1 is rotated about the center axis 0--0'.
The reference numeral 4 denotes an optical head for applying laser light to the vertically magnetized film 3 of the magneto-optical disk 1. This optical head 4 is comprised chiefly of a semiconductor laser 5 and a condensing lens 6. The optical head 4 is designed to act on the vertically magnetized film 3 so that the laser light may be focused thereon, and to be movable radially with respect to the magneto-optical disk 1. The reference numeral 7 designates an electromagnet disposed so as to be opposed to the optical head 4 relative to the magneto-optical disk 1 for applying a bias magnetic field to the vertically magnetized film 3. This electromagnet 7 is designed to move radially of the magneto-optical disk in operative association with the optical head 4. An electric current inverted in phase in conformity with a recording signal is supplied from a magnetic field modulation circuit 9 to the coil 8 of the electromagnet 7.
The magnetic field modulation circuit 9 is operated by a recording signal input from a terminal 10. For example, when the recording signal changes as shown in (b.sub.1) of FIG. 2A of the accompanying drawings, the electromagnet 7 produces a magnetic field shown in FIG. 2D in synchronism therewith. Here, when the recording signal is "1", a magnetic field of a predetermined intensity +H is produced in a direction with the direction of magnetization of the vertically magnetized film 3 as the upward direction. On the other hand, when the recording signal is "0", a magnetic field of a predetermined intensity -H is produced just in the opposite direction to that when the recording signal is "1". At this time, the optical head 4 continuously emits a laser light of a predetermined intensity as shown in FIG. 2C when the recording condition is brought about, and the laser light locally illuminates the vertically magnetized film of the magneto-optical disk 1. Thus, the temperature of the vertically magnetized film in this irradiated portion rises to the curie point or higher.
By the process as described above, a recording pattern as shown as a model in FIG. 2E is formed on the vertically magnetized film. In this case, the blank portions and the recording patterns indicated by hatching show the upwardly magnetized state and the downwardly magnetized state, respectively.
In the above-described method, however, the spot of the light beam has a finite size, and this has led to a problem that the end portion of each bit of the recording pattern suffers from positional deviation relative to the recording signal, i.e., so-called bit shift, as shown in FIG. 2. On the other hand, the shape of the recording bit becomes a chevron-like shape as shown in FIG. 2E and therefore, there has been created a time difference t.sub.2 between the center of the recording signal and the peak of a signal (shown in FIG. 2F) reproduced from this recording pattern. This phenomenon will hereinafter be referred to as the peak shift. Due to this phenomenon, for example, the time t.sub.1 to the peak of the reproduction signal (FIG. 2F) becomes shorter by (t.sub.3 -t.sub.2) relative to the time t.sub.0 from a synchronizing signal reproduced from a prepit formed unevenly on the magneto-optical disk 1 to the center of "1" of the recording signal. Here, t.sub.3 is the time difference by bit shift, and t.sub.2 is the time difference by peak shift.